Magnesium oxide is an inorganic compound excellent in heat conductivity, heat resistance and the like, and can be mixed to various resins and used as a heat conductive filler for improving the thermal conductivity and fire resistance of a resin composition or a resin molded article obtained by molding the same. In molded articles obtained by immobilizing magnesium oxide powder with a binder such as monolithic refractories, molded articles of a resin kneaded matter obtained by filling the resin with magnesium oxide as a filler and the like, weather resistance and shape stability are influenced by volume stability of magnesium oxide forming them.
The reason for the poor volume stability of magnesium oxide is that the magnesium oxide undergoes a hydration reaction to become magnesium hydroxide, and the volume change (expansion) occurs at this time. That is, since magnesium oxide has a relatively high hydration property, the volume tends to expand due to water absorption to cause cracking of the resin, which has been an obstacle for putting magnesium oxide into practical use as a thermally conductive filler. Therefore, when magnesium oxide is used as a thermally conductive filler, a technique for enhancing hydration resistance has been required.
Conventionally, in order to enhance the hydration resistance of magnesium oxide, a technique for coating the surface of magnesium oxide with a surface treatment agent such as silica or phosphoric acid has been known. However, such a coating technique requires a treatment cost because a surface treatment agent is used.
On the other hand, a technique for improving the hydration resistance of magnesium oxide without using a surface treatment agent is also known. For example, Patent Literature 1 describes high purity magnesia clinker in which the surface of the particles of magnesia clinker is covered with fine crystals of magnesium hydroxide, thus digestion of periclase by water vapor in water or air is suppressed (that is, excellent in hydration resistance). Moreover, it is described in the examples and comparative examples of this literature that the magnesia clinker is sized, then added with water, and kept in a thermostat at a predetermined temperature, but in either example, the temperature is 30° C. (Example 1, and the like), or 60° C. or more (Example 4, and the like).
Incidentally, although it relates to magnesium oxide particles for tablet production, Patent Literature 2 describes magnesium oxide particles of which surface is covered with a coating layer including magnesium hydroxide and/or magnesium carbonate. Further, it is also described that the magnesium oxide particles are produced by bringing water vapor or a mixed gas of water vapor and carbon dioxide gas into contact with magnesium oxide particles. Also, in the examples of this literature, a coating layer is formed by introducing steam into the magnesium oxide particles and holding at 95° C. for 2 hours, or a coating layer is formed by introducing steam and holding at 90° C., then introducing carbon dioxide gas and holding for 3 hours.